Iron Enhancing Superoxide-Mediated Mn(II) Oxidation by Peroxymonosulfate: Elucidating the Role of Superoxide Radicals

The effective removal of soluble Fe(II) and Mn(II) is problematic in water supply utilities. This study explored the oxidation behavior, kinetics, and reaction mechanisms of using peroxymonosulfate (PMS) to mediate the co-oxidation of Fe(II) and Mn(II) in natural water. At [Fe(II)] and [Mn(II)] of 1 mg/L, PMS oxidized all Fe(II) spontaneously within 15 s, irrespective of the oxidant concentration (50–500 μM) and solution pH (6–9), while it required 7–30 min for complete Mn(II) oxidation, indicating its distinctive behavior in reacting with Fe(II) and Mn(II). Scavenging assays and electron paramagnetic resonance (EPR) analysis revealed the dominant presence of O₂^•– in the system. EPR analysis combined with chemical probing experiments using nitroblue tetrazolium chloride suggested that O₂^•– was produced exclusively via surface reactions of ferric oxide with PMS. PMS co-oxidation eventually yielded amorphous hydrous manganese-bearing ferric co-oxides (hMnFeO_x), with increasing Mn:Fe compositional ratios over time and pH, i.e., Mn_0.31Fe_0.69, Mn_0.67Fe_0.33, Mn_0.93Fe_0.07 at pH 7 and Mn_0.68Fe_0.32, Mn_0.89Fe_0.11, Mn_0.90Fe_0.10 at pH 9. The co-occurrence of Fe(II) provided hydrous FeO_x surfaces enriched with chemisorbed oxygen (∼60%), acting as nucleation sites for the heterogeneous MnO_x oxidation through enhanced electron transfer and surface complexation pathways. This co-occurrence thus reduced the half-life time of PMS-induced Mn(II) oxidation, by 5.3–18.7 times compared to the Mn(II) oxidation alone. This study provides fresh evidence, underscoring the significance of O₂^•– in PMS-mediated metal oxidation systems.